Method for depositing porous films

ABSTRACT

A processing method for depositing porous silica and doped silica films is provided. The method uses a cyclic scheme wherein each cycle comprises first codepositing silica with silicon, then selectively removing the silicon to form a porous structure. In a preferred embodiment, the codeposition is carried out by plasma enhanced chemical vapor deposition. The reagent feed stream comprises a mixture of codeposition reagents and a selective silicon removal reagent. RF power modulation is used to control the codeposition and the selective silicon removal steps with the later proceeds whenever the RF power is turned off or reduced to a low level. A porous film with highly uniform small pores and a desired porosity profile can be obtained with this method. This method is advantageous for forming a broad range of low-k dielectrics for semiconductor integrated circuit fabrication. The method is also advantageous for forming other porous films for other applications.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates generally to a processing method fordepositing porous films on a substrate. More specifically, the presentinvention relates to a processing method for depositing porous silica ordoped silica films for fabricating semiconductor integrated circuits.The method is also advantageous for use in other applications whereporous structures are required.

2. Description of Related Art

Traditionally, silicon dioxide, having a dielectric constant (k) about4, is used as the insulator material for fabricating semiconductorintegrated circuits. As device dimensions shrink, interconnect RC(resistance-capacitance) delay issues require the insulator to have alower dielectric constant in order to deliver superior circuitperformance. The semiconductor industry has identified these targets atvarious technology nodes and published them on the InternationalTechnology Roadmap for Semiconductors. Dielectric constant less than 4is commonly referred to as low-k and that less than 2.2 is commonlyreferred to as ultralow-k. It is projected that beyond the 90 nm devicegeneration, low-k dielectrics having a k-value below 2.6 is desirablefor device fabrication.

The dielectric constant is a measure of the tendency of a material toallow an externally applied electric field to induce electric dipoles inthe material. This so-called electric polarizability is governed by theelectronic, ionic and distortion polarization in the material. A goodreview of the polarization phenomena and a more detailed description ofthe various classifications of low dielectric constant materials can befound in the article by K. Maex et al. [K. Maex, M. R. Baklanov, D.Shamiryan, F. Iacopi, S. H. Brongersma, and Z. S. Yanovitskaya, J. App.Phys., Vol. 93, No. 11, p. 8793-8841] or the chapter by S. Wolf[“Silicon Processing for The VLSI Era, Volume 4: Deep-Submicron ProcessTechnology” by S. Wolf, Lattice Press, Sunset Beach, Calif., 2002, p.639-670].

Fundamentally, to weaken the polarization in silicon dioxide, one canalter the structural lattice of silicon and oxygen, replace some or allof the silicon-oxygen bonds with less polarized bonds, and/or introducefree space to decrease material density in the film. Explored effortsinclude developing 1) silica-based doped oxides, 2) silsesquioxane-basedinorganic-organic hybrid polymers, 3) organic polymers, and 4) amorphouscarbon films.

Silica-based doped oxides are usually deposited by chemical vapordeposition (CVD) methods with or without plasma enhancement. Fluorinedoping provides fluorosilicate glass (FSG) with a k-value about 3.6.Carbon or other alkyl substitution reduce the dielectric constantfurther; some reaching k-values as low as 2.6 to 2.8. An altogetheramorphous carbon film or a fluorocarbon film has been reported to yieldlower k-values. However, amorphous carbon technology is still veryimmature and for now is not ready for manufacturing considerations.

CVD silica-based doped oxides are appealing for use as semiconductordielectrics due to their silicon-oxide like structure. The films requirealmost no modification in circuit designs. Semiconductor manufacturerscan also leverage existing toolsets and infrastructures to continuetheir device fabrication. Some of these films have been adopted at the180 nm, 150 nm, 130 nm, and even 90 nm nodes. However, the oxycarbidefilms are prone to carbon depletion in subsequent processing, resultingin a less than desired final dielectric constant. Furthermore, theincorporation of carbon in silica introduces many process complications,particularly in etching, chemical mechanical polishing, and cleaning.Consequently, implementation has been formidable and costly.

In contrast, silsesquioxane-based inorganic-organic hybrid polymers andorganic polymers are inherently low-k dielectrics due to their moreopened molecular lattice than silicon dioxide and less polarized bondsin the molecular components. These materials can provide a broad rangeof low k values. These films are usually applied by spin coating,although some can also be deposited by CVD methods. The spun film mustgo through curing to drive off excess solvent, complete the chemicalreactions, and undergo densification. Compared to silicon dioxide, thesefilms are generally mechanically softer and less thermally stable. Theyalso tend to take up moisture so additional cap layers are oftenrequired to protect them. Because of the different properties, there aremany restrictions in conventional processing and modifications arefrequently needed to accommodate these films in process integration.Therefore, widespread adoption has not been noted.

Recently, the industry has come to conclude that there is no fully densespin-on or CVD material that has a low enough dielectric constant and atthe same time satisfy all the diverse requirements for robustintegration for the 90 nm generation and beyond. Since the dielectricconstant scales proportionally with the host matrix density, attentionhas been turned to exploring the viability of reducing the dielectricconstant by introducing porosity in the insulator.

Sol-gel techniques are known to provide a flexible means forincorporating dopants and forming a porous template in silica networks.Sol-gel techniques, however, require meticulous gellation and drying.Their different modes of processing, process control, and integrationschedules are incompatible with semiconductor device manufacturing. Manyof these films also exhibit deterioration of mechanical properties withdecreasing k-values.

A more adaptable approach to introduce void volume in the dielectricshas been the use of sacrificial porogens [see for example, U.S. Pat. No.6,271,273 and U.S. Pat. No. 6,451,712]. A thermally unstable material,referred to as the porogen, is blended with an organosilicate polymerand applied to form a film as in conventional spin-on dielectrics. Thefilm is cured, then subject to an annealing step to volatize the porogenwhile forming a skeletal porous framework of the cured film. Critical tothis thermolytic technique are: First, the porogen must separate fromthe thermosetting matrix and must decompose as well as removed entirelyduring the annealing step without leaving behind any residue. Second,the porogen decomposition must take place below the host's glasstransition temperature without bringing about any collapse of the porousstructure. Third, change in film stress must be carefully managed duringphase separation and thermal expulsion of porogens without causing anyfilm cracking or delamination. Porous films formed with this methodusually have a broad pore size distribution with the smallest pores inthe 20 nm range.

The porogen concept has also been explored with CVD techniques [see forexample, U.S. Pat. No. 6,054,206 and U.S. Pat. No. 6,171,945]. Thermallyunstable labile organic groups are deposited in organosilicate glass.The film is then annealed to volatize the labile organic components,resulting in a porous structure. An alternative e-beam treatment [seefor example, U.S. Pat. No. 6,737,365] or ultraviolet exposure [see forexample, U.S. Pat. App. 20040096672] has also been reported to beeffective in removing these species and additionally enhancecross-linking of the host material. In general, nanoporous films withpores commensurate in size of the departing organic groups are obtainedwith this approach. The nanoporous matrix is claimed to provide goodmechanical and thermal stability in subsequent processing. However, likeother porogen techniques, this CVD technique is based on volatilizingorganic species, and there are concerns for residual outgassing if theorganic species that are supposed to be removed are not removedentirely. Moreover, process integration complications associated withprocessing carbon-containing oxide films still remain, as discussedearlier.

To date, development of low-k films continues. The object of thisinvention is to create a CVD process method for generating a porouslow-k dielectric film that is extendable to the ultralow-k range. It isdesirable that the film is chemically, mechanically, and thermallystable, similar to that of silicon dioxide. It is further desirable thatprocess integration requirements are not excessive and costly whencompared to established techniques.

BRIEF SUMMARY OF THE INVENTION

The present invention is generally directed to forming porous films.More specifically, the present invention is directed to forming poroussilica or doped silica films on a substrate for fabricatingsemiconductor integrated circuit. The disclosed method uses a cyclicprocess scheme to deposit the film. In each cycle, a thin layer ofsilica and silicon or doped silica and silicon is first codeposited.(The codeposited film may be commonly viewed as a silicon-rich oxide orsilicon-rich doped oxide). Then, the film is exposed to a chemicalreagent that can preferentially remove silicon over silica in thecodeposit, leaving behind a porous structure. The processing steps arerepeated alternately to build up the thickness of the film.

The pore size and pore distribution in each layer are determined by theamount of sacrificial silicon incorporated and how the silicon isdispersed in that layer. Each subsequent deposition step puts down alayer of codeposit on the previously created porous layer and thefollowing selective silicon removal step develops the porous structurein-situ. Hence, by means of the cyclic method disclosed in thisinvention, a desired porosity profile can advantageously be obtained inthe silica film by tailoring the processing conditions at each cycle.

There are three salient features in this invention: 1) the codepositionof the silica (or the host matrix material in general) with silicon, 2)the exposure of the codeposit film to a selective silicon removalreagent during the silicon removal step, and 3) a methodology tofacilitate and optimally control the codeposition and selective siliconremoval steps.

As an embodiment of this invention, the selective silicon removalreagent is advantageously selected from a group of molecular halides orhalogenated species comprising fluorine, chlorine, bromine and theirderivatives thereof. The selective silicon removal reagent can also be avapor derived from a solution containing potassium hydroxide, ortetramethylammonium hydroxide (TMAH), or ethylene diamine pyrocatecol(EDP), or their like or derivatives thereof, optionally mixed with ahigh vapor pressure carrier gas such as an alcohol that do not reactwith the vapor.

As a further embodiment of this invention, the preferred reagent tofacilitate the selective silicon removal reaction is selected from thegroup consisting of molecular fluorine, xenon difluoride, and theircombinations thereof.

In a preferred implementation of this invention, the deposition step iscarried out by plasma enhanced chemical vapor deposition (PECVD)techniques. The reagent stream comprises a codeposition mixture thatcomprises at least one silicon-containing precursor and other additionalchemical reagents known to those skilled in the arts for facilitatingPECVD of silica and silicon or doped silica and silicon, and at leastone selective silicon removal reagent.

In the preferred method, RF power modulation is used to facilitate thecyclic process of codeposition and selective silicon removal. When theRF power is at an optimal level for deposition of silica and silicon,deposition of the materials proceed. When the RF power is turned off orreduced to a low value, no dissociation occurs, whether dissociationleading to codeposition or dissociation of the silicon removal reagent.At this time, by means of the chemical actions of the silicon removalreagent, silicon is preferentially removed from the codeposit, leavingbehind a porous silica structure.

The porous silica film obtained with this method has uniformly dispersedsmall size pores, commensurate with the uniformity and distribution ofsilicon dispersed in the codeposit. The pore size and porosity profilein the film is determined by the reactor chamber design, flow rates ofthe components in the reagent mixture, deposition conditions such astemperature, pressure, RF power, electrode spacing, and parametersrelating to how the cyclic process is conducted such as the processcycle frequency and the duty cycle.

Other embodiments of the invention are disclosed in the claims. Thepresent invention is generally applicable also for forming a porousdoped silica film provided that the dopant constituents do not reactwith the selective silicon removal reagent significantly, or should itreact, the residual reaction products are benign or desirable forenhancing the properties of the film. Likewise, the dopants areincorporated into the silica during the codeposition. The invention isfurther generally applicable for forming porous films of any host matrixmaterial that can codeposit with silicon and is relatively chemicallyinert to the selective silicon removal reagent.

The process method described herein provides a means to obtain a porouslow-k dielectric film for fabricating semiconductor integrated circuits.The method is also advantageous for fabricating other porous structuresfor other applications in fields including, but not limiting to,semiconductor, advanced packaging, energy storage, and advancedMicrosystems.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the present invention can be readily understood byconsidering the detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 is a flow diagram illustrating the sequential alternating stepsof silica-silicon codeposition followed by selective removal of thesilicon in the codeposit layer to form a porous silica film. After eachcycle, the process conditions can be changed to tailor the porosityprofile development in the film.

FIGS. 2A to 2I schematically illustrate the development of an exemplaryporous film according to the cyclic codeposition and selective siliconremoval process in this invention.

FIG. 3A to 3I schematically illustrate the development of an exemplaryporous film with a varied porosity profile that can be obtained with thecyclic process of this invention.

FIG. 4 is a schematic defining the process cycle frequency, cycleperiod, duty cycle, and illustrating the RF power level of an exemplaryRF power delivery waveform.

FIG. 5 is a flow diagram illustrating a generic cyclic process schemefor forming other porous films based on repeating alternating steps ofsilicon codeposition with the film material followed by selectivesilicon removal. This generic process is claimed as a broader scope ofthis invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a process method for forming poroussilica or doped silica films on a substrate for semiconductor integratedcircuits fabrication. The method is also generally applicable forforming porous films of other host matrix materials using the featuresdescribed in the disclosure. For simplicity in discussion, we shallfocus the description primarily on porous silica films.

According to the invention, the method entails a cyclic process schemeto form the film. In each cycle, a thin layer of silica and silicon ordoped silica and silicon are first codeposited. (The codeposited filmmay be commonly viewed as a silicon-rich oxide or silicon-rich dopedoxide). Then, the silicon is selectively removed from the codeposit tocreate a porous silica structure. The processing steps are repeatedalternately to build up the thickness of the film. In this description,the conditions and implementation of the invention are further detailed.

For clarity, we shall refer herein “silicon” in the film as any looselybonded silicon to silica, such as interstitial silicon or silicon bondedwith hydrogen or hydroxyl, or any silicon covalently bonded to silicon,or elemental silicon, that is incorporated in the film during thecodeposition. We shall refer “silica” in the film as any oxidizedsilicon that contains silicon fully or partially bonded to oxygen. Inaddition, the term “silica” is used interchangeably to refer to alltypes of oxidized silicon, undoped or doped with other constituents,unless otherwise stated. Silicon-rich oxide is regarded as a codepositof silica and silicon. The term “host matrix” refers to the solid matterof a porous structure and “host matrix material” refers to the materialthat made up the porous structure.

FIG. 1 shows a simplified flow diagram of the cyclic process and FIGS.2A to 2I are schematic cross-sectional views of the processed film atdifferent stages of film formation. These cross-sectionalrepresentations are simplified for illustrative purposes only and shouldnot be taken as actual arrangement of the constituents in the codepositfilm.

The process starts at step 102 with a substrate in the reaction chamber.In FIG. 2A, the starting substrate is illustrated as a silicon wafer 202that have undergone some previous processing. The result from previousprocessing is schematically represented by a combined structure 204 onthe silicon wafer. The surface of 204 is typical starting surface forprocessed wafers entering the stage of low-k dielectric deposition insemiconductor device fabrication. The surface 204 can alternatively beanything for other applications, including the bare surface of a siliconwafer.

At step 104, initial process parameters are set to codeposit a thinlayer of silica and silicon. At step 106, deposition is conducted andproduces a codeposit film 220 that comprises dispersed silicon 206 insilica 208. This is shown schematically in FIG. 2B.

In the next step 108, silicon 206 is selectively removed from thecodeposit 220. FIG. 2C shows a represented cross-section of the openedsilica matrix 230 with only silica 208 remaining.

Usually more than one cycle is required to form the desired filmthickness, so step 106 and 108 are repeated sequentially in the nextcycle. By step 106 in the next cycle, the same codeposited layer 220forms atop the previously created opened silica matrix layer 230,enclosing the opened spaces to form pores 215, as shown in FIG. 2D. FIG.2E shows that after the subsequent selective removal step 108, thesilicon 206 in the codeposit 220 is removed. The repeated deposition andselective removal steps now form a thicker porous silica film.

Step 110 in FIG. 1 determines if the desired film thickness has beenachieved. If not, the same sequence of codeposition and selectivesilicon removal steps are repeated to build up the thickness of theporous film. FIGS. 2A to 2I shows illustratively the result fromcompletion of four cycles. Many more cycles can be repeated as desired.The process ends in step 120.

The pore size and pore distribution in each layer is determined by thedispersion of silicon in the codeposit and the amounts of sacrificialsilicon incorporated. In light of this characteristic, the cyclicprocess is also flexible for obtaining a porous film with a variedporosity profile. FIGS. 3A to 3I illustrates schematically a variedporosity film that can be obtained on the same starting surface 204.Layer 238 is schematically shown to contain less silicon than Layer 220and after the silicon removal step 108, Layer 238 will form a porouslayer 240 that is less porous than Layer 230. As a note, the porosityprofile depicted in FIGS. 3A to 3I is intended for illustrative purposesonly and does not limit the various profiles that can be obtained withthis invention.

As shown in the flow diagram in FIG. 1, after each cycle, the processconditions can be reselected at step 114 to put down a different silicaand silicon codeposit film. A different amount of silicon incorporatedwill alter the porosity of the layer. FIG. 3B and FIG. 3I show that adense silica film 250 can be formed if so desired anywhere along thethickness of the porous film. In that case, no silicon incorporation isrequired in the codeposition step.

Since the porous film is formed completely in-situ, as anotherembodiment of this invention, any additional fabrication processingsteps can be inserted before or after the cyclic process as desired.Like layer 250 illustrated in FIG. 3I, a similarly dense liner layer,cap layer, etch stop layer, or any other process layer can be depositedtogether with the porous film without breaking vacuum. This isparticularly advantageous for process integration, for example, in theprocess of forming a dual damascene structure.

For this invention, there is no porogen to volatize post deposition. Theporous silica matrix is chemically similar to dense silicon dioxide. Thefinely distributed pores provide good structural integrity and thermalstability. A broad range of low-k dielectrics can be obtained with thismethod.

Of essence in this invention are 1) the codeposition of silica (or thehost matrix material in general) with silicon, 2) the exposure of thecodeposit film to a selective silicon removal reagent during theselective silicon removal step, and 3) facilitation of the cyclicprocessing to modulate the codeposition and the selective siliconremoval step.

The use of silicon as the sacrificial material to form the porous filmis a critical aspect in this invention. The codeposition with siliconprovides a silica template from which the porous structure develops. Theselective silicon removal reagent enables the structure to be developed.There are certain chemical reagents well known to the silicon industrythat can chemically react readily with silicon but not with many othermaterials. Hence, we can utilize these differential chemical reactivityproperties to facilitate our selective silicon removal step.

For this invention, the selective silicon removal reagent isadvantageously selected from a group of molecular halides or halogenatedspecies comprising fluorine, chlorine, bromine and their derivativesthereof. The selective silicon removal reagent can also be selected fromvapors derived from solutions containing potassium hydroxide, ortetramethylammonium hydroxide (TMAH), or ethylene diamine pyrocatecol(EDP), or their like or derivatives thereof, optionally mixed with ahigh vapor pressure carrier gas such as an alcohol that do not reactwith the vapor.

For this invention, the preferred selective silicon removal reagent isselected from a group consisting of molecular fluorine (F₂), xenondifluoride (XeF₂), and their combinations thereof.

Another important aspect of this invention is the facilitation of thecyclic processing to modulate the codeposition and the selective siliconremoval step. Depending on the desired properties of the film, there area number of ways to implement this invention. Those skilled in the artsunderstand alternative methods of deposition and numerous forms topractice the cyclic processing without departing from the spirit of thisinvention.

In a preferred embodiment of this invention, the codeposition isprovided using plasma enhanced chemical vapor deposition techniques. APECVD reactor with a radio frequency (RF) source at 13.56 MHz of asymmetric parallel plate configuration is used to produce plasma fordeposition in a process chamber. The reagent stream comprises acodeposition reagent mixture and one or more selective silicon removalreagent. Intermittent RF power is used to regulate the codeposition andthe selective silicon removal steps. The same reagent stream is suppliedto the process chamber during both of these steps.

FIG. 4 illustrates the important features in the RF power modulationcontrol. For simplicity of illustration, the RF power waveform isrepresented as a rectangular pulse train in this diagram. In practice,other waveforms can also be used. The waveform is characterized by acycle frequency, a cycle period, and a duty cycle. The cycle period isthe duration of time for the completion of all the processing steps inone process cycle. The cycle frequency is the inverse of the cycleperiod, and the duty cycle is the proportion of time within the cycleperiod during which codeposition occurs.

In FIG. 4, we show the process cycle consists of the codeposition andthe selective silicon removal steps. During codeposition, the RF poweroperates at P_(d). At this power level, the RF power is high enough todissociate the precursors for the codeposition of both silica andsilicon. For the rest of the cycle, the RF power level is turned off orset to a low level such that deposition is discouraged. Not only mustthe power level be low enough so that the precursors for silica andsilicon deposition cannot dissociate, but must also be below P_(f), thepower level that can dissociate the selective silicon removal reagent.FIG. 4 arbitrary depicts the waveform the power level set at zero duringthe selective silicon removal step.

To facilitate simultaneous PECVD silica and silicon formation, thecodeposition reagent mixture must comprise at least onesilicon-containing precursor and one oxygen-containing precursor. Theselection of the precursors shall not be belabored here since thoseskilled in the arts of PECVD techniques understand various combinationsof chemicals that are suitable. However, we will include the followingcodeposition mixture in the preferred process for this invention. Thepreferred codeposition mixture comprises silane (SiH₄), nitrous oxide(N₂O), with or without tetraethylorthosilicate (TEOS), with or withouthydrogen (H₂), and argon (Ar) or other inert gas.

As is in the case of conventional silicon-rich PECVD silicon oxide, thesilica-silicon codeposit will contain silica and some interstitialsilicon atoms, silicon covalently bonded to each other, and siliconweakly bonded to hydrogen or hydroxyl groups or the like. When thiscodeposit is exposed to the selective silicon removal reagent, thereagent will chemically etch the silicon while the silica will remainvirtually intact. The chemical reaction will proceed at a ratedetermined by the process temperature and the concentration of theselective silicon removal reagent in the process chamber.

In our preferred implementation, the same reagent mixture iscontinuously fed into the reaction chamber during the codeposition andthe selective silicon removal steps. Accordingly, the codepositionreactions will occur in the presence of the selective silicon removalreagent. With RF power on, the selective silicon removal reagent willdissociate and participate in the plasma reactions. It may even beincorporated in the film. For example, if fluorine is used as theselective silicon removal reagent, some fluorinated oxide will also beformed during codeposition. (The fluorinated oxide will be beneficialfor further lowering the dielectric constant of the silica). Inaddition, depending on the codeposition process conditions, theseradicals will etch away some of the deposited silicon and oxide. In viewof all these reactions, while a reasonable concentration of theselective silicon removal reagent must be present in the feed stream tofacilitate the selective silicon removal at a reasonable rate, it isimportant to keep this concentration low relative to the codepositionconditions such that the net result is deposition during thecodeposition step.

Besides the deposition and the silicon removal process conditions, thecycle frequency and the duty cycle also play a role in the developmentof the porous film. The duration of the selective silicon removal stepis usually set long enough to remove some or all the silicon depositedin the same cycle. If too thick a layer is deposited in a cycle, thenthe silicon removal reagent will have to work through a thick codepositto remove the silicon. It is not efficient, and sometimes it may not beeffective even for a small molecule like fluorine, to diffuse far into amaterial to react. On the other hand, depositing a thin layer may notalways be desirable either, especially if large silicon incorporation isdesired to construct a high porosity film. Too thin a layer will resultin a sparse silica matrix after the silicon removal. The codeposit inthe next cycle may fill up the gaps. Thus, the process cycle frequencyand the duty cycle must be set in relation to the codeposition andselective silicon removal rates such that a desired pore size andporosity profile of the film can be obtained.

As another embodiment, the cyclic method presented in this invention canbe used to engineer porous silica or doped silica films with nanosizepores. The capability is illustrated in the following example. Ifsilicon is codeposited at an apparent¹ rate of 400 Å/min and silica iscodeposited at an apparent rate of 1000 Å/min, and if the process cyclefrequency is 3 Hz with 50% duty cycle (i.e., the codeposition durationis 0.167 sec), then within each cycle, only a few angstroms of thecodeposit is formed. Under such condition, the size of the dispersedsilicon atoms will be on the same order as the thickness of thecodeposit layer. This will give rise to dispersed openings of the samesize when the silicon atoms are selectively removed. The codepositformed in the next cycle will cover these openings and enclose the spacebeneath. The method will produce a film with advantageous nanosizepores. Thus, porous films with pore sizes ranging from 0.3 nm to 50 nmand porosity ranging from 0.5% to 90% can be obtained with this method.¹ The apparent deposition rate can be calculated from the relativeamount of silica and silicon deposited in the film within a given timeinterval. For example, the relative amount of silica and silicon in thecodeposit layer can be estimated from the relative amount of silicon andoxygen in the deposit determined from SIMS (secondary ion massspectroscopy) analysis and FTIR (Fourier Transform Infrared) absorptionspectra.

In summary of the conditions to practice the PECVD process describedabove, we include the following further embodiments. The preferredreagent stream comprises silane, nitrous oxide, with or withouttetraethylorthosilicate (TEOS), and with or without hydrogen or argon orother inert gas. The silane to nitrous oxide flow ratio is between 0.005and 100. The feed stream also contains 0.1% to 50% molecular fluorine orxenon difluoride or their combinations thereof. During deposition, thePECVD chamber is maintained at a pressure between 0.01 torr and 15 torrwith the electrode spacing between 0.1 inch and 3 inches, the substratetemperature between 25° C. and 500° C. and the RF power density between0.01 W/cm² and 5 W/cm². The selective silicon removal step is performedat the same pressure and temperature as the codeposition step. The 13.56MHz RF power is delivered at a process cycle frequency from 0.0005 Hz to500 Hz with codeposition duty cycle ranging from 1% to 99%. Forclarification, here the RF power density of a symmetric parallel platereactor is defined as the RF power divided by two times the area of thecathode or anode.

More preferably, the PECVD process is conducted with a feed streamcontaining silane to nitrous oxide in a flow ratio from 0.01 to 30. Thefeed stream also contains 1% to 30% of molecular fluorine, xenondifluoride, or their combinations thereof. During deposition, thechamber pressure is maintained between 0.1 torr and 10 torr with theelectrode spacing between 0.3 inch and 1.5 inches, substrate temperatureheld between 300° C. and 400° C., and RF power density between 0.2 W/cm²and 1.0 W/cm². The 13.56 MHz RF power is delivered at a process cyclefrequency from 0.1 Hz to 10 Hz with codeposition duty cycle ranging from5% to 70%.

Needless to say, this preferred implementation of the present inventioncould be practiced readily on a PECVD system equipped with RF powermodulation control. If RF power modulation control is not available onthe PECVD system, one can use a series of process recipe steps tosimulate the RF power switching during the cyclic process.Alternatively, one can modify the RF power delivery hardware with atiming circuit or modify the system to provide a means such that the RFpower delivered for deposition can intermittently be switched off orreduced to a low level as previously stated. As an option, the siliconremoval reagent and the codeposition reagent mixture can be introducedtogether as a single reagent feed stream for the codeposition and thesilicon removal steps.

Another way to implement the present invention on a conventional PECVDreactor is to run the codeposition and the selective silicon removalsteps separately as two independent process programs. These twoprocesses will be performed alternately in the same chamber to simulatethe cyclic processing. While the codeposition is performed as aconventional PECVD process, no RF power is applied during the selectivesilicon removal step. The codeposition reagents and the selectivesilicon removal reagents can be supplied separately during theirrespective processes.

Alternatively, in chamber designs that have multiple stages forsequential processing [see for example, U.S. Pat. No. 6,007,675],another way to implement the present invention is to transfer thesubstrate to a separate stage in the same reactor after the codepositionstep. RF power is applied only to the stage that is undergoingcodeposition. In this scenario, at least two wafers, one undergoingcodeposition and one undergoing selective silicon removal, will beprocessed together at the same time in the same reactor with the samefeed stream but on different stages.

Still another way to implement the present invention is to conduct thecodeposition and the selective silicon removal steps in separatereaction chambers in a cluster tool. The substrate is transferredbetween two chambers so that it receives sequential processing ofcodeposition and selective silicon removal.

With isolated chamber processing, any deposition methodology that canform the codeposit can be used to practice the cyclic process of thepresent invention. Some methodologies may be physical vapor deposition,thermal chemical vapor deposition, spin coating and others. In fact, theimplementation can be extended to any setup so long as a silica-siliconcodeposit is formed and the codeposit is exposed to an ambientcontaining the selective silicon removal reagent to remove the silicon.The same process sequence can be repeated as desired to develop thethickness of the porous film.

In all these foregoing implementations, it is important to note thatthey are not all equivalent in allowing the same range of process cyclefrequency and duty cycle to be performed. Therefore, the applicabilityof each implementation depends on the particular process chemistry andprocess conditions desirably selected. We believe using RF amplitudemodulation in a PECVD configuration provides the most flexible meanswith stable controls of the codeposition and the selective siliconremoval environment for efficient practice of this invention.

Finally, the description presented herein does not distinguish themethod for formation of a porous silica film or formation of a porousdoped silica films such as porous fluorine-doped silica (FSG), porouscarbon-doped silica, porous phosphorous-doped silica (PSG), poroushydrogen silsesquioxane (HSQ), porous methyl silsesquioxane (MSQ),porous boron-doped silica (BSG), porous boron-phosphorus-doped silica(BPSG), or the like. In fact, it does not distinguish the method forformation of other porous films such as porous silicon nitride, poroussilicon oxynitride, porous silicon carbide, porous boron nitride, porousboron oxynitride, porous aluminum oxide, porous aluminum nitride, porousaluminum oxynitride, or the like. To reiterate, salient in thisinvention are the codeposition with silicon and the selective removal ofthe sacrificial silicon. Hence, the scope of this invention covers notonly formation of porous silica films, but also other porous films ofhost matrix materials, inorganic or organic or a combination thereof,that are relatively inert to the selective silicon removal reagent. Evenif some of the constituent film materials react with the selectivesilicon removal reagent, as long as a porous film is formed, thepractice is within the scope of the present invention.

As an example, a porous carbon-doped silica film can be formed with thepresent invention by codepositing carbon-containing species in thesilica-silicon film. The use of selective silicon removal reagent likefluorine will attack some of the carbon in the film to form fluorocarbonspecies during the silicon removal step. Depending on the filmconstituents and the process conditions, the reaction products may bevolatile and can be removed together with the silicon fluorides or theymay be non-volatile and leave behind a fluorocarbon component in thefilm. (Note that C—F bonds are beneficial for providing hydrophobicproperty of the film). The desirability of the film is to be determined,but as long as the essential features of silicon codeposition andselective silicon removal is followed to form the porous film, thepractice fall within the scope of this invention.

Henceforth, in FIG. 5, we present further a generic flow diagram for theformation of porous films using the basic concept of this invention.Similar to the process flow shown in FIG. 1, but instead of step 106,the codeposition incorporates all the constituent film materials andsilicon in step 506. In step 508, the selective silicon removal reagentremoves silicon from the codeposit. Other constituents in the film mayalso react with the selective silicon removal reagent but theconstituents that make up the desired host matrix material should remainrelatively inert to the selective silicon removal reagent. The sameembodiments that apply to the silica films apply to these cases.

In summary, the present invention can be practiced to provide poroussilica and doped silica films with a broad range of low dielectricconstants for semiconductor integrated circuit fabrication. The samecyclic processing method can be extended more generally to provide otherporous films using silicon as the sacrificial material and using atleast one of the selective silicon removal reagents to remove thesilicon to form the porous film. The method is advantageous for use inmany applications including and not limiting to semiconductor, advancedpackaging, energy storage, and advanced Microsystems.

While the above description is directed to the embodiments of thepresent invention, other and further embodiments of the invention may bedevised without departing from the basic scope thereof. For example, theuse of a plasma generated from low RF frequency in the kilohertz rangeor from frequency range in the megahertz other than 13.56 MHz, the useof mixed frequency RF power, or different ways of generating the plasma,whether capacitively or inductively coupled, or decoupled, should all beregarded as different adaptations of the same PECVD implementationembodied in this invention. It is further to be noted that variations ofthe cyclic process, such as addition of any pre- and post-depositiontreatment comprising plasma, electron beam, ion beam, ultraviolet,chemical, or thermal processing steps to activate the exposed filmsurfaces or modify the film properties do not depart from the spirit ofthis invention. The deposition of additional materials to enhance thefilm properties or assist the silicon removal reaction also does notdepart from the spirit of this invention.

1. A method based on plasma enhanced chemical vapor depositiontechniques for depositing a porous film of a host matrix material on asubstrate in a vacuum environment, comprising: (a) Using a reagentmixture stream that comprises at least one silicon-containing precursor,at least one selective silicon removal reagent, and other additionalchemical reagents known to those skilled in the arts for facilitatingthe PECVD of the film host matrix material and the PECVD of silicon; (b)Using RF power modulation to facilitate the execution of a plurality ofprocessing cycles to form the desired thickness of the porous film,wherein each cycle comprises codeposition of the host matrix materialwith silicon when the RF power level is set at a level suitable for thecodeposition of the film host matrix material and silicon, and selectiveremoval of silicon from the codeposit to form the porous structure bychemical exposure of the codeposit to the reagent mixture containing theselective silicon removal reagent when the RF power level is turned offor set below that which is required for deposition or the dissociationof the selective silicon removal reagent.
 2. The method of claim 1wherein the film host matrix material is an inorganic or an organicmaterial or a combination thereof.
 3. The method of claim 1 wherein thefilm host matrix material is a member of the group consisting of silica,carbon-doped silica, fluorine-doped silica (FSG), boron-doped silica(BSG), phosphorus-doped silica (PSG), boron-phosphorus-doped silica(BPSG), germanium-doped silica (GSG), hydrogen silsesquioxane (HSQ),methyl silsesquioxane (MSQ), silicon nitride, silicon oxynitride,silicon carbide, aluminum oxide, aluminum nitride, aluminum oxynitride,boron nitride, boron oxynitride, and combinations thereof.
 4. The methodof claim 1 wherein the selective silicon removal reagent is selectedfrom a group consisting of molecular halides and halogenated speciescomprising fluorine, chlorine, bromine, and their derivatives thereof.5. The method of claim 1 wherein the selective silicon removal reagentis selected from the group consisting of molecular fluorine, xenondifluoride, and their combinations thereof.
 6. The method of claim 1wherein the selective silicon removal reagent is a vapor derived from asolution containing at least a chemical selected from the groupconsisting of potassium hydroxide, tetramethylammonium hydroxide (TMAH),ethylene diamine pyrocatecol (EDP), and their derivatives thereof. 7.The method of claim 6 wherein the vapor is further mixed with a highvapor pressure carrier gas that is chemically inert to the selectivesilicon removal reagent.
 8. The method of claim 1 wherein the PECVDconditions comprise RF excitation frequency ranging from 100 kHz to 100MHz in a single frequency mode, preferably at 13.56 MHz.
 9. The methodof claim 1 wherein the PECVD conditions comprise RF excitation frequencyranging from 100 kHz to 100 MHz in a mixed frequency mode.
 10. Themethod of claim 1 carried out in a PECVD system equipped with RF powermodulation capability.
 11. The method of claim 1 carried out in a PECVDsystem modified to provide a means to enable control of RF power levelchange to facilitate cyclic execution of the codeposition step, whichrequires RF power, and the selective silicon removal step, which doesnot require RF power but can accommodate a level of RF power below thatis required for the dissociation of the selective silicon removalreagent.
 12. The method of claim 1 wherein the RF power is delivered ata process cycle frequency from 0.0005 Hz to 500 Hz with codepositionduty cycle ranging from 1% to 99%, preferably at a process cyclefrequency from 0.1 Hz to 10 Hz with codeposition duty cycle ranging from5% to 70%.
 13. The method of claim 1 wherein the codeposition RF powerdensity is between 0.01 W/cm² and 5 W/cm², preferably between 0.2 W/cm²and 1.0 W/cm².
 14. The method of claim 1 wherein the process isperformed with electrode spacing held between 0.1 inch and 3 inches,preferably between 0.3 inch and 1.5 inches.
 15. The method of claim 1wherein the process is performed with the substrate temperature heldbetween 25° C. and 500° C., preferably between 300° C. and 400° C. 16.The method of claim 1 wherein the process is performed with the chamberpressure maintained between 0.01 torr and 15 torr, preferably between0.1 torr and 10 torr.
 17. The method of claim 1 wherein the reagentmixture comprises silane and nitrous oxide in the flow ratio between0.005 to 100, preferably between 0.01 to
 30. 18. The method of claim 17wherein the reagent mixture further comprises argon.
 19. The method ofclaim 17 wherein the reagent mixture further comprises hydrogen.
 20. Themethod of claim 1 wherein the concentration of the selective siliconremoval reagent in the reagent mixture is between 0.1% and 50%.
 21. Themethod of claim 1 wherein the reagent mixture comprises molecularfluorine in a concentration between 1% and 30%.
 22. The method of claim1 for making a porous film with a desired porosity profile byindividually selecting the processing conditions (including temperature,pressure, electrode spacing, deposition RF power, process cyclefrequency, process cycle period and duty cycle) for some of thecodeposition and selective silicon removal steps.
 23. The method ofclaim 1 for making a porous film with pore sizes ranging from 0.3 nm to50 nm.
 24. The method of claim 1 for making a porous film with porosityfrom 0.5% to 90%.
 25. The method of claim 1 further comprises at leastone additional treatment step performed at selected cycle interval ofthe process, wherein the treatment step is selected from the groupcomprising plasma, electron beam, ion beam, electromagnetic radiation,chemical, or thermal exposure.
 26. The method of claim 1 furthercomprises depositing a liner layer on the substrate prior to depositingthe porous film without breaking vacuum.
 27. The method of claim 1further comprises depositing on top of the porous film a capping layerwithout breaking vacuum.
 28. The method of claim 1 further comprisesdepositing on top of the porous film a low k etch stop layer withoutbreaking vacuum.
 29. A method for depositing a porous film of a hostmatrix material on a substrate in a vacuum environment, comprising aplurality of processing cycles, wherein each cycle comprises: (a)Codepositing the host matrix material with silicon by plasma enhancedchemical vapor deposition in a PECVD process chamber using acodeposition reagent mixture stream that comprises at least onesilicon-containing precursor and other additional chemical reagentsknown to those skilled in the arts for facilitating the PECVD of thefilm host matrix material and the PECVD of silicon; (b) In a subsequentstep, exposing the codeposit to a reagent stream that comprises aselective silicon removal reagent so that silicon is preferentiallyremoved from the codeposit by the chemical actions of the selectivesilicon removal reagent, leaving behind a porous structure of the hostmatrix material; whereby repeated execution of the codeposition andselective silicon removal steps build up the thickness of the porousfilm.
 30. The method of claim 29 wherein the film host matrix materialis an inorganic or an organic material or a combination thereof.
 31. Themethod of claim 29 wherein the film host matrix material is a member ofthe group consisting of silica, carbon-doped silica, fluorine-dopedsilica (FSG), boron-doped silica (BSG), phosphorus-doped silica (PSG),boron-phosphorus-doped silica (BPSG), germanium-doped silica (GSG),hydrogen silsesquioxane (HSQ), methyl silsesquioxane (MSQ), siliconnitride, silicon oxynitride, silicon carbide, aluminum oxide, aluminumnitride, aluminum oxynitride, boron nitride, boron oxynitride, andcombinations thereof.
 32. The method of claim 29 wherein the selectivesilicon removal reagent is selected from a group consisting of molecularhalides and halogenated species comprising fluorine, chlorine, bromine,and their derivatives thereof.
 33. The method of claim 29 wherein theselective silicon removal reagent is selected from the group consistingof molecular fluorine, xenon difluoride, and their combinations thereof.34. The method of claim 29 wherein the selective silicon removal step isa vapor derived from a solution containing at least a chemical selectedfrom the group consisting of potassium hydroxide, tetramethylammoniumhydroxide (TMAH), ethylene diamine pyrocatecol (EDP), and theirderivatives thereof.
 35. The method of claim 34 wherein the vapor isfurther mixed with a high vapor pressure carrier gas that is chemicallyinert to the selective silicon removal reagent.
 36. The method of claim29 wherein the PECVD conditions comprise RF excitation frequency rangingfrom 100 kHz to 100 MHz in a single frequency mode, preferably at 13.56MHz.
 37. The method of claim 29 wherein the PECVD conditions comprise RFexcitation frequency ranging from 100 kHz to 100 MHz in a mixedfrequency mode.
 38. The method of claim 29 wherein the codeposition andthe selective silicon removal steps are carried out in separate processchambers in the same cluster tool.
 39. The method of claim 29 whereinthe codeposition and the selective silicon removal steps are carried outin the same process chamber.
 40. The method of claim 29 carried out in aPECVD system using recipe programs, including a series of processingsteps in recipe programs, to facilitate cyclic execution of thecodeposition and the selective silicon removal steps.
 41. The method ofclaim 29 carried out on separate stages of a multistage reactor whereinthe codeposition step is conducted on one stage by PECVD with RF power,and the selective silicon removal step on a different stage without RFpower.
 42. The method of claim 29 wherein the selective silicon removalstep is performed at a temperature between 25° C. and 500° C.,preferably between 300° C. and 400° C.
 43. The method of claim 29wherein the selective silicon removal step is performed at chamberpressure maintained between 0.01 and 700 torr, preferably between 0.1torr to 10 torr.
 44. The method of claim 29 wherein the codepositionstep is performed with the substrate temperature held between 25° C. and500° C., preferably between 300° C. and 400° C.
 45. The method of claim29 wherein the codeposition step is performed with the chamber pressuremaintained in the range between 0.01 torr and 15 torr, preferablybetween 0.1 torr and 10 torr.
 46. The method of claim 29 wherein thecodeposition step is performed with electrode spacing held between 0.1and 3 inches, preferably between 0.3 and 1.5 inches.
 47. The method ofclaim 29 wherein the codeposition step is performed with the RF powerdensity between 0.01 and 5 W/cm², preferably between 0.2 and 1.0 W/cm².48. The method of claim 29 wherein the reagent stream for thecodeposition step comprises silane and nitrous oxide in a flow ratiobetween 0.005 and 100, preferably between 0.01 and
 30. 49. The method ofclaim 48 wherein the reagent stream for the codeposition step furthercomprises argon.
 50. The method of claim 48 wherein the reagent streamfor the codeposition step further comprises hydrogen.
 51. The method ofclaim 29 wherein the concentration of the selective silicon removalreagent in the process chamber is between 0.1% and 100%.
 52. The methodof claim 29 wherein the reagent stream for the selective silicon removalstep comprises molecular fluorine in a concentration between 0.1% and100%.
 53. The method of claim 29 for making a porous film with a desiredporosity profile by individually selecting the processing conditions(including temperature, pressure, electrode spacing, and deposition RFpower) for some of the codeposition and selective silicon removal steps.54. The method of claim 29 for making a porous film with pore sizesranging from 0.3 nm to 50 nm.
 55. The method of claim 29 for making aporous film with porosity from 0.5% to 90%.
 56. The method of claim 29further comprises at least one additional treatment step performed atselected cycle interval, wherein the treatment step is selected from thegroup comprising plasma, electron beam, ion beam, electromagneticradiation, chemical, or thermal exposure.
 57. The method of claim 29further comprises depositing a liner layer on the substrate prior todepositing the porous film without breaking vacuum.
 58. The method ofclaim 29 further comprises depositing on top of the porous film acapping layer without breaking vacuum.
 59. The method of claim 29further comprises depositing on top of the porous film a low k etch stoplayer without breaking vacuum.
 60. A method for depositing a porous filmof a host matrix material on a substrate, comprising a plurality ofprocessing cycles, wherein each cycle comprises: (a) Codepositing thehost matrix material with silicon in one step; (b) In a subsequent step,exposing the codeposit to a reagent stream that comprises at least aselective silicon removal reagent so that silicon is preferentiallyremoved from the codeposit by the chemical actions of the selectivesilicon removal reagent, leaving behind a porous structure of the hostmatrix material; whereby repeated execution of the codeposition andselective silicon removal steps build up the thickness of the porousfilm.
 61. The method of claim 60 wherein the codeposition step isconducted with a deposition technique selected from the group comprisingspin coating, thermal chemical vapor deposition, physical vapordeposition, and deposition techniques assisted by electromagnetic energyranging from radio frequency to microwave spectrum.
 62. The method ofclaim 60 wherein the film host matrix material is an inorganic or anorganic material or a combination thereof.
 63. The method of claim 60wherein the film host matrix material is a member of the groupconsisting of silica, carbon-doped silica, fluorine-doped silica (FSG),boron-doped silica (BSG), phosphorus-doped silica (PSG),boron-phosphorus-doped silica (BPSG), germanium-doped silica (GSG),hydrogen silsesquioxane (HSQ), methyl silsesquioxane (MSQ), siliconnitride, silicon oxynitride, silicon carbide, aluminum oxide, aluminumnitride, aluminum oxynitride, boron nitride, boron oxynitride, andcombinations thereof.
 64. The method of claim 60 wherein the selectivesilicon removal reagent is selected from a group consisting of molecularhalides and halogenated species comprising fluorine, chlorine, bromine,and their derivatives thereof.
 65. The method of claim 60 wherein theselective silicon removal reagent is selected from the group consistingof molecular fluorine, xenon difluoride, and their combinations thereof.66. The method of claim 60 wherein the selective silicon removal reagentis a vapor derived from a solution containing at least a chemicalselected from the group consisting of potassium hydroxide,tetramethylammonium hydroxide (TMAH), ethylene diamine pyrocatecol(EDP), and their derivatives thereof.
 67. The method of claim 66 whereinthe vapor is further mixed with a high vapor pressure carrier gas thatis chemically inert to the selective silicon removal reagent.
 68. Themethod of claim 60 wherein the codeposition and the selective siliconremoval steps are carried out in separate process chambers in the samecluster tool.
 69. The method of claim 60 wherein the selective siliconremoval step is performed at a temperature between 25° C. and 500° C.,preferably between 300° C. and 400° C.
 70. The method of claim 60wherein the selective silicon removal step is performed at chamberpressure maintained between 0.01 torr and 700 torr, preferably between0.1 torr and 10 torr.
 71. The method of claim 60 wherein theconcentration of the selective silicon removal reagent in the reagentstream is between 0.1% and 100%.
 72. The method of claim 60 wherein thereagent stream for the selective silicon removal step comprisesmolecular fluorine in a concentration between 0.1% and 100%.
 73. Themethod of claim 60 for making a porous film with a desired porosityprofile by individually selecting the processing conditions in some ofthe codeposition and selective silicon removal steps.
 74. The method ofclaim 60 for making a porous film with pore sizes ranging from 0.3 nm to50 nm.
 75. The method of claim 60 for making a porous film with porosityfrom 0.5% to 90%.
 76. The method of claim 60 further comprises at leastone additional treatment step performed at selected cycle interval,wherein the treatment step is selected from the group comprising plasma,electron beam, ion beam, electromagnetic radiation, chemical, or thermalexposure.
 77. The method of claim 60 further comprises depositing aliner layer on the substrate prior to depositing the porous film withoutbreaking vacuum.
 78. The method of claim 60 further comprises depositingon top of the porous film a capping layer without breaking vacuum. 79.The method of claim 60 further comprises depositing on top of the porousfilm a low k etch stop layer without breaking vacuum.